1. Field of the Invention
The present invention relates to an optical recording material, an optical recording medium and an optical record reproducing device, and, particularly, to a large scale volume type optical recording medium, optical recording materials and raw materials thereof, such as a photo-responsive high-molecular compound, a photo-responsive high-molecular composition, a dicarboxylic acid monomer and a polyester, which are used in the optical recording medium and an optical record reproducing device which records and reproduces information by using the optical recording medium.
2. Description of the Related Art
Rewritable optical disk recording devices such as phase change types and photomagnetic types have been widely spread already. However, these optical disks are not admitted to have performances coping with future demands for large scale recording to deal with an increase in scale along with a development of highly functional operating systems (OS) and application software, a trend to a huge scale on account of the spread of multimedia for various documents and data for presentation and digital recording of video signals of a long time animated cartoon with high precision and high density. In such a high density and large scale optical disk recording device, in order to increase recording density the ideas that, for example, the diameter of a beam spot is decreased to shorten an interval between neighboring trucks or neighboring bits are exploited.
DVD-ROMs are among those put to practical use by the development of such technologies. These DVD-ROMs store 4.7 Gbyte data on one surface of a disk 12 cm in size. Rewritable and erasable DVD-RAMs enables recording with a density as high as 5.2 Gbyte on both surfaces of a disk 12 cm in diameter by a phase change system. Namely, the DVD-RAM enables writing and reading of information with a capacity four times that of a CD-ROM or with a capacity corresponding to 1900 or more disks in the case of floppy disks. Like this, the optical disks are progressing in densification every year. However, on the other hand, because the aforementioned optical disks record data in a plane, the diffraction limit of light limits the recording, leading to the physical limit of high density recording. In order to develop a larger scale disk, three-dimensional (volume type) recording including recording in the direction of the depth is required.
As a volume type optical recording medium as mentioned above, a photorefractive material medium enabling volume recording of a holographic grating and the like are regarded as promising ones. Among photorefractive materials (hereinafter called “PR material”), those are known which have high sensitivity and therefore absorb relatively weak light on the same level as that of a solid laser with changing in refractive index. These photorefractive materials are expected to be applied to volume multiple hologram recording with the possibility that super-high density and super-large scale can be attained.
To state the principle of the photorefractive effect, two coherent light waves are applied to a PR material to form interference. An electron at a dobor level is excited to a conductive band in a place where light intensity is strong, moved by diffusion or drift and caught in a place where light intensity is weak. A positive charge remains in the place where light intensity is strong and a negative charge remains in the place where light intensity is weak. This allows a charge distribution to be formed, producing static electric field. A change in refractive index is caused as a result of the electro-optical effect of the static electric field. The period of the change in refractive index is the same as that of the interference fringe and the index modulation functions as a holographic grating.
As the PR material, inorganic ferroelectric crystals such as barium titanate, lithium niobate and bismuth silicate (BSO) have been frequently used. These materials produce a highly sensitive and highly efficient photo-induced index changing effect (photorefractive effect), but on the other hand, have drawbacks that many of them have a difficulty in crystal growth and are so hard and fragile that they are not processed into desired shapes and also have a difficulty in controlling their sensitive wavelengths.
In recent years, PR materials comprising organic materials have been proposed as those which overcome these drawbacks. Generally, organic PR materials comprise i) a charge generating material which receives light to produce a charge, ii) a charge transfer material which promotes the transfer of the generated charge in a medium, iii) a dichroic organic dye sensitive to the induced electric field by the charge transfer, iv) a high-molecular base material (binder) which supports these materials and v) additives (e.g., a plasticizer and a compatibility-improving agent) which change the physical properties of base materials. Also, there is the case where one component combines plural functions as, for example, a material doubling as a charge transfer material and a high-molecular base material and a material doubling as a charge transfer material and a plasticizer. The effects of these organic PR materials cause a positive and a negative charge to generate from the charge generating material which has absorbed light. These charges are separated into a positive charge and a negative charge by the charge transfer material when existing external electric field is effected, whereby an internal electric field is created. The internal electric field causes a change in the orientation of the dichroic dye, which causes a change in the distribution of refractive index in the base material. If such an organic PR material is applied, volume hologram recording with high recording density is considered to be possible in theory.
The organic PR material, however, has the problem that it is essential to apply an external electric field in substance. The electric field is as remarkably large as several hundreds V•mm−1 which imposes a large restriction on devices when using the system as a recording device. Moreover, few different materials such as the charge generating material, the charge transfer material and the high-molecular base material are mixed and used in the material system, giving rise to a large problem concerning a reduction in stability caused by phase separation during recording or storage.
To avoid the foregoing problem, for instance, S. Hvilsted et al., proposes that using a polymer having cyanoazobenzene at a side chain, an index modulation is written therein to record holograms [Opt, Lett., 17 [17], 12 (1992)]. It has been clarified that in the material, an index modulation with 2500 high and low refractive indexes can be written in a space of 1 mm and it is expected that high recording density will be attained.
A holographic memory of a layer of a polymer having azobenzene at a side chain utilizes the photo-induced anisotropy of the polymer layer. Azobenzene in an amorphous azo polymer layer takes a random orientation state. When linearly polarized and excited light having a wavelength which corresponds to the absorption band belonging to the π-π* transition of an azo group is applied to the azo polymer layer, azobenzene as a trans isomer is excited in such a high probability that the transition dipole moment accords to the direction of the polarized light, namely, selectively, and is eventually photo-isomerized to a cis isomer. The excited cis isomer is isomerized again to a trans isomer by light or heat.
A change in the orientation of azobenzene is caused in a direction stable to the excited light, namely a direction perpendicular to the direction of the polarization through such an angle-selective trans-cis-trans isomerization cycle caused by the application of polarized light. Because azobenzene has an optical anisotropy, it exhibits birefringence and dichroism as a result of a change in orientation. Utilizing the photo-induced anisotropy makes it possible to record a hologram by the distribution of intensity and the distribution of polarization. Because the recording is based on a change in the orientation of the polymer, it is carried out stably for a long period of time and a hologram can be recorded repeatedly by erasing the recorded hologram by applying circularly polarized light or by applying heat to an isotropic phase. As a material for a rewritable type holographic memory, a layer of the polymer having azobenzene at a side chain is a most promising material type.
For instance, the inventors of the invention have proposed polyesters having azobenzene, which is useful as an optical recording material, at the side chain in Japanese Patent Application Laid-Open (JP-A) Nos. 2000-109719, 2000-264962 and 2001-294652. In JP-A No. 2000-109719, a monomer and a polyester in which a methyl group is introduced into azobenzene to control the absorption band to within a range suitable for optical recording and optical recording media using these monomer and polyester are disclosed. Also, in JP-A No. 2000-264962, a polyester which is made suitable to optical recording by defining a methylene chain as a main chain and by controlling the glass transition temperature of a polymer and optical recording media using the polyester are proposed. In addition, in JP-A No. 2001-294652, it is disclosed that optical recording characteristics is improved using a polyester in which a methylene chain as a side chain is defined.
“A development of thick recording media” is most important to attain the development of a large scale volume type holographic memory. Generally, the condition of incident angle necessary for diffraction becomes more strict with an increase in the thickness of a hologram. Namely, diffracted light diminishes only by a small deviation of the incident condition from the Bragg's condition. The angle multiplex system in the volume type holographic memory makes use of the angular selectivity. Namely, plural holograms are formed in the same volume and the incident angle of reading light is controlled whereby a desired hologram can be read out without any crosstalk. If the angle selectivity is improved by increasing the thickness of a recording medium in this manner, multiplicity can be raised and recording capacity can be increased accordingly.
Also, the magnitude of modulation of refractive index for forming a hologram is limited by the capability of a medium material. For this, the formation of plural holograms in the same volume corresponds to the fact that the capability of modulation of refractive index which a material possesses is divided by plural holograms upon use. Because the square of the amplitude of refractive index acts on diffraction efficiency, a rise in multiplicity reduces the diffraction efficiency of a hologram in inverse proportion to the square of the multiplicity. Accordingly, it is desired to develop recording media which can obtain a high diffraction efficiency raised to some extent also in the case of raising the multiplicity.
On the other hand, in the case of the layer of a polymer having azobenzene at a side chain, it is necessary to record using a wavelength enough to excite the π-π* transition of azobenzene. Although it is effective to select wavelengths which can be highly absorbed to improve recording sensitivity, this gives rise to another problem at the same time. Namely, if a material which highly absorbs light with a recording wavelength is used, incident recording light is absorbed by molecules in the vicinity of the surface, with the result that an effective hologram cannot be formed over a whole region extending in the direction of the thickness of a medium. Further, the absorption loss of the medium makes it difficult to attain high diffraction efficiency. Accordingly, in order to accomplish a rise in thickness with maintaining high recording sensitivity and high diffraction efficiency, it is important to control the absorbance of a medium (the amount of light to be absorbed) for recording wavelengths.
Also, the magnitude and stability of the photo-induced anisotropy (birefringence) of the layer of a polymer having, at a side chain, azobenzene to be a source of the formation of a hologram are largely affected by the thermal properties of the polymer. In general, the photo-induced birefringence of an amorphous polymer is relatively small and has inferior record retentivity. On the contrary, the photo-induced birefringence of a crystalline or liquid crystalline polymer is relatively large, is stable to heat and has superb record retentivity. However, these crystalline and liquid crystalline polymer layers are increased in thickness, noises due to scattering caused by the crystal are increased, posing the problem that errors are produced when reading out data. From such a problem, the thickness of the film of a polymer having azobenzene at a side chain has been limited to about the order of 20 μm to 40 μm when applying it to a holographic memory. Accordingly, it is important to control the crystallinity of the polymer to attain an increase in layer thickness while maintaining high record retentivity and preventing the generation of noises.
The invention has been made in view of the aforementioned prior art problem and it is an object of the invention to provide an optical recording material (e.g., a photo-responsive high-molecular compound, a photo-responsive high-molecular composition or a polyester) which can be increased in layer thickness while maintaining high recording sensitivity and high diffraction efficiency by controlling the absorbance of a medium and also to provide its raw material (dicarboxylic acid monomer). Another object of the invention is to provide an optical recording material which can be increased in the layer thickness while maintaining high record retentivity and preventing the generation of noises by controlling the crystallinity of the material and also to provide its raw material. Also, a further object of the invention is to provide an optical recording medium enabling large scale recording by accomplishing an increase in layer thickness without damaging recording characteristics. A still further object of the invention is to provide an optical record reproducing device enabling recording and reproduction of large scale data.